This invention relates to an electronic flash and a camera provided with an electronic flash.
In recent years, there has been a tendency to make the main body of a compact camera provided with an electronic flash smaller and more compact. Accordingly, a charging device of the flash has been forced to have a smaller main capacitor for storing an energy to be emitted from the flash. Further, in order to speed up a photography preparation in flash photographing, it is required to charge the main capacitor at a higher speed.
In order to meet the above requirements, in the charging device of the known electronic flash, the main capacitor is made smaller by reducing a withstand voltage as much as possible and charging is performed at a high speed by accurately detecting a charge voltage of the main capacitor and accurately controlling a charge stop timing based on this charge voltage. More specifically, the lower the withstand voltage, the smaller the main capacitor can be made. Accordingly, by making a margin for the charge voltage as small as possible to reduce the withstand voltage of the main capacitor, the main capacitor is made smaller. Further, the high speed charging of the main capacitor is performed by increasing a wind number ratio (wind number of secondary coil/wind number of primary coil) of a step-up transformer of the charging device to perform the charging at as large a charge current as possible while accurately controlling the charge stop timing.
Further, in the charging device of the prior art flash, the excessive charging of the main capacitor and the abnormal generation of heat within the charging device need to be prevented in the case that a charge stop control becomes impossible due to a certain abnormality. Accordingly, if charging is continued for longer than a specified time, a safety measure is taken to forcibly stop the charging operation upon the judgment of a charging abnormality.
A charging time differs depending upon a variety of conditions such as a battery voltage, a residual voltage of the main capacitor and an ambient temperature and is not constant in every charging operation. Thus, a time by which the main capacitor is excessively charged and an amount of heat generated within the charging device in an abnormal manner when an abnormality occurs differ.
However, in the charging device of the prior art flash, the charging operation is not forcibly stopped unless the time fixedly set in advance for the protection against excessive charging elapses. Therefore, in some cases, the main capacitor may be damaged by excessive charging before this control for the protection from against charging starts, and/or the heat abnormally generated within the charging device may exceed a limit value, with the result that the protection function may not effectively operate.